American Fibertek MT-915C-SL - Manual (Page 6)

6
OFFSET BIAS – RS485
The RS485 specification requires receivers to detect input signals down to 200mVp-p of voltage level.
In many cases this can cause systems to be sensitive to noise on the data wires. In an effort to
eliminate the effects of low levels of noise, some manufacturers of equipment that communicate using
RS485 have introduced a small voltage bias to the data lines. This is usually accomplished using a 470
Ohm resistance to +5V on the positive line and 470 Ohm resistance to ground on the negative line.
When used in conjunction with the appropriate termination resistors referred to in the previous section,
this introduces about a 300 mV offset, improving noise immunity.
MT-915C-SL STATUS INDICATORS
The MT-915C-SL transmitter provides the following LED status indicators to aid in installation and
troubleshooting:
DATA TX/RX INDICATORS
DATA TX and DATA RX indicators are provided to monitor each of the two available data channels.
DATA 1 TX and RX correspond with the multi-protocol input/output of DATA CH 1. DATA 2 TX and RX
correspond with the contact closure input/output of DATA CH 2.
DATA 1 TX
A green LED indicator is provided to monitor the data coming in from the electrical interface, through
the MT-915C-SL, and out onto the fiber. The intensity of this indicator will vary with input data patterns;
however in typical applications it will cycle on and off as data is transmitted. Data transmitted status
associated with this LED is summarized below.
DATA 1 TX LED Data Status
Green Data Flow Present
Off Data Flow Not Detected
DATA 1 RX
A green LED indicator is provided to monitor the data coming in from the fiber, through the MT-915C-
SL, and out onto the electrical interface. The intensity of this indicator will vary with input data patterns;
however in typical applications it will cycle on and off as data is received. Data received status
associated with this LED is summarized below.
DATA 1 RX LED Data Status
Green Data Flow Present
Off Data Flow Not Detected
DATA 2 TX
A green LED indicator is provided to monitor the contact input from the electrical interface, through the
MT-915C-SL, and out onto the fiber. Contact status associated with this LED is summarized below.
Data 2 TX LED Contact Status
Green Contact Input Closed
Off Contact Input Open